Absorbent liquid for CO2 and/or H2S, and apparatus and method using same

ABSTRACT

An absorbent liquid which absorbs at least one of CO 2  and H 2 S from a gas, including a secondary linear monoamine; a tertiary linear monoamine or a sterically hindered primary monoamine; and a secondary cyclic diamine, wherein a concentration of each of the secondary linear monoamine, the tertiary linear monoamine or the sterically hindered primary monoamine; and the secondary cyclic diamine is less than 30% by weight.

TECHNICAL FIELD

One or more embodiments of the present invention relate to an absorbentliquid for CO₂ (carbon dioxide) and/or H₂S (hydrogen sulfide), and anapparatus and a method using the same, and in particular to an absorbentliquid for CO₂ in combustion exhaust gas and an apparatus and a methodfor removing CO₂.

BACKGROUND

A variety of methods have conventionally been proposed concerning themethod of removing acid gases, in particular, CO₂, which are containedin gases produced at chemical plants and power plants such as naturalgas and synthetic gas and combustion exhaust gas. Examples of such amethod include a method of removing CO₂ and H₂S from combustion exhaustgas by contact with an aqueous solution of an alkanolamine as anabsorbent liquid.

Taking as an example an absorbent liquid of monoethanolamine (MEA) beinga primary monoamine among alkanolamines, for example, the aboveabsorbent liquid itself deteriorates due to oxygen and the like in thecombustion exhaust gas. Thus, there are known an absorbent liquid formedby blending a secondary monoamine with a secondary cyclic diamine or aparticular primary monoamine having a high steric hindrance effect (forexample, Patent Document 1), an absorbent liquid formed by adding atertiary monoamine to a mixture of a secondary monoamine and a secondarycyclic diamine (for example, Patent Documents 2 and 3), and an absorbentliquid formed by mixing a particular primary monoamine having a highsteric hindrance effect, a secondary monoamine, and a tertiary monoamine(for example, Patent Document 4).

REFERENCE DOCUMENT LIST Patent Documents

-   Patent Document 1: Japanese Patent No. 5215595-   Patent Document 2: Japanese Patent No. 4634384-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. 2013-086079-   Patent Document 4: Japanese Unexamined Patent Application    Publication No. 2008-168227

However, in the case of the components and their blend ratio for theabsorbent qliquids disclosed in Patent Documents 1 to 4, the reboilerfor regenerating the absorbent liquid requires a large amount of heat.

SUMMARY

Considering the above circumstances, one or more embodiments of thepresent invention aim to provide an absorbent liquid for CO₂ and/or H₂S,and an apparatus and a method using the same, which make it possible toreduce the reboiler heat duty when regenerating the absorbent liquid.

According to one or more embodiments of the present invention, anabsorbent liquid which absorbs CO₂ and/or H₂S in a gas, includes: (a) asecondary linear monoamine; (b1) a tertiary linear monoamine or (b2) asterically hindered primary monoamine; and (c) a secondary cyclicdiamine, in which a concentration of each of the components is less than30% by weight.

According to one or more embodiments of the present invention, in theabsorbent liquid as described herein, the concentration of (c) thesecondary cyclic diamine is less than the concentration of (a) thesecondary linear monoamine and is less than the concentration of (b1)the tertiary linear monoamine or (b2) the primary monoamine in weightpercent relative to the absorbent liquid.

According to one or more embodiments of the present invention, in theabsorbent liquid as described herein, the concentration of (b1) thetertiary monoamine or (b2) the sterically hindered primary monoamine isthe same as or less than the concentration of (a) the secondary linearmonoamine in weight percent relative to the absorbent liquid.

According one or more embodiments of the present invention, in theabsorbent liquid as described herein, a total concentration of theconcentration of (a) the secondary linear monoamine and theconcentration of (b1) the tertiary linear monoamine or (b2) the primarymonoamine is 20 to 55% by weight or less.

According to one or more embodiments of the present invention, in theabsorbent liquid as described herein, a total concentration of (a) thesecondary linear monoamine, (b1) the tertiary linear monoamine or (b2)the primary monoamine, and (c) the secondary cyclic diamine is 70% byweight or less.

According to one or more embodiments of the present invention, in theabsorbent liquid as described herein, the component (a) secondary linearmonoamine is a compound represented by Formula (I) below:

wherein, R1 is a hydrocarbon group having 1 to 4 carbon atoms, and R2 isa hydroxyalkyl group having 1 to 4 carbon atoms.

According to one or more embodiments of the present invention, in theabsorbent liquid as described herein, (b1) the tertiary linear monoamineis a compound represented by Formula (II) below:

wherein, R3 is a hydrocarbon group having 1 to 4 carbon atoms, and R4and R5 are each a hydrocarbon group or a hydroxyalkyl group having 1 to4 carbon atoms.

According to one or more embodiments of the present invention, in theabsorbent liquid as described herein, (b2) the primary monoamine is acompound represented by Formula (III) below:

wherein, R6 to R8 are each a hydrocarbon group or a hydroxyalkyl grouphaving 1 to 4 carbon atoms.

In addition, according to one or more embodiments of the presentinvention, in the absorbent liquid as described herein, (c) thesecondary cyclic diamine is a piperazine derivative.

Moreover, according to one or more embodiments of the present invention,an apparatus for removing CO₂ and/or H₂S, includes: the absorbent liquidas described herein; an absorption tower which absorbs CO₂ and/or H₂Sfrom the gas; and a regeneration tower which regenerates the absorbentliquid containing the CO₂ and/or H₂S absorbed therein by heat of areboiler.

Furthermore, according to one or more embodiments of the presentinvention, a method for removing CO₂ and/or H₂S, includes: an absorptionstep of absorbing CO₂ and/or H₂S by bringing the absorbent liquid asdescribed herein into contact with the gas; and a regeneration step ofregenerating the absorbent liquid containing the CO₂ and/or H₂S absorbedtherein by heat of a reboiler.

According to one or more embodiments of the present invention, byblending (a) a secondary linear monoamine, (b1) a tertiary linearmonoamine or (b2) a sterically hindered primary monoamine, and (c) asecondary cyclic diamine at a predetermined ratio, it is possible toprovide an absorbent liquid for CO₂ and/or H₂S, and an apparatus and amethod using the same, which make it possible to reduce the amount ofreboiler heat when regenerating the absorbent liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an apparatus for removing CO₂and/or H₂S according to one or more embodiments of the presentinvention.

FIG. 2 is a graph illustrating reboiler heat duty ratios of examples anda comparative example of an absorbent liquid according to the presentinvention.

FIG. 3 is a graph illustrating a reboiler heat duty ratio with respectto an amine component concentration of the examples and the comparativeexample of the absorbent liquid according to the present invention.

FIG. 4 is a graph illustrating reboiler heat duty ratios of examples anda comparative example of the absorbent liquid according to the presentinvention.

FIG. 5 is a graph illustrating a reboiler heat duty ratio with respectto an amine component concentration of the examples and the comparativeexample of the absorbent liquid according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, a detailed description is provided for one or moreembodiments of an absorbent liquid for CO₂ and/or H₂S, and an apparatusand a method using the same according to the present invention. Thepresent invention is not limited to the embodiment described later.

A description is provided for the absorbent liquid according to one ormore embodiments of the present invention. The absorbent liquid asdescribed herein contains components of (a) a secondary linearmonoamine, (b1) a tertiary linear monoamine, and (c) a secondary cyclicdiamine Additionally, the concentration of each of the components of thesecondary linear monoamine, the tertiary linear monoamine, and thesecondary cyclic diamine is less than 30% by weight relative to theweight percent of the absorbent liquid. The total concentration of thesecondary linear monoamine, the tertiary linear monoamine, and thesecondary cyclic diamine is preferably 70% by weight or less and morepreferably 10% by weight or more relative to the absorbent liquid. Theabsorbent liquid is formed by dissolving and mixing the above componentsin water, and the main constituent other than the above components iswater.

The upper limit of the concentration of the secondary linear monoamine(a) is less than 30% by weight and is preferably 28% by weight or less.In addition, the lower limit of the concentration is more preferably 10%by weight or more and is further preferably 15% by weight or more. Theupper limit of the concentration of the tertiary linear monoamine (b1)is less than 30% by weight, preferably 28% by weight or less, andfurther preferably 25% by weight or less. In addition, the lower limitof the concentration is preferably 10% by weight or more and morepreferably 15% by weight or more. The upper limit of the concentrationof the secondary cyclic diamine (c) is less than 30% by weight,preferably within a range of 15% by weight or less, and more preferably10% by weight or less. In addition, the lower limit of the concentrationis preferably 1% by weight or more and more preferably 5% by weight ormore.

The total concentration of the secondary linear monoamine and thetertiary linear monoamine relative to the absorbent liquid is preferably20 to 55% by weight and more preferably 25 to 50% by weight. Inaddition, the concentration of the tertiary linear monoamine ispreferably the same as or less than the concentration of the secondarylinear monoamine relative to the weight percent of the absorbent liquid.In particular, the concentration of the tertiary linear monoamine ismore preferably 0.6 to 1 in a weight ratio relative to the concentrationof a secondary linear amine. The concentration of the secondary cyclicdiamine is preferably less than the concentrations of the secondarylinear monoamine and the tertiary linear monoamine relative to theweight percent of the absorbent liquid.

As described above, if (a) the secondary linear monoamine, (b1) thetertiary linear monoamine, and (c) the secondary cyclic diamine are atthe above concentrations, an excellent CO₂ absorbing property of (a) thesecondary linear monoamine and (c) the secondary cyclic diaminemaintains the CO₂ absorbability of the absorbent liquid, and anexcellent CO₂ stripping property of (a) the secondary linear monoamineand (b1) the tertiary linear monoamine can improve the CO₂ strippingperformance of the absorbent liquid. Thus, it is possible to reduce thereboiler heat duty when regenerating the absorbent liquid which hasabsorbed CO₂.

The secondary linear monoamine is a compound represented by Formula (I)below. In Formula (I), R1 represents a hydrocarbon group having 1 to 4carbon atoms, and R2 represents a hydroxyalkyl group having 1 to 4carbon atoms.

In particular, the secondary linear monoamine is at least one selectedfrom the group consisting of N-(methylamino)ethanol (C₃H₉NO),N-(ethylamino)ethanol (C₄H₁₁NO), N-(propylamino)ethanol (C₅H₁₃NO), andN-(butylamino)ethanol (C₆H₁₅NO).

The tertiary linear monoamine is a compound represented by Formula (II)below. In Formula (II), R3 represents a hydrocarbon group having 1 to 4carbon atoms, R4 represents a hydrocarbon group or a hydroxyalkyl grouphaving 1 to 4 carbon atoms, and R5 represents a hydrocarbon group or ahydroxyalkyl group having 1 to 4 carbon atoms.

In particular, the tertiary linear monoamine is at least one selectedfrom the group consisting of N-methyldiethanolamine (C₅H₁₃NO₂),N-ethyldiethanolamine (C₆H₁₅NO₂), N-butylethanolamine (C₈H₁₉NO₂),4-dimethylamino-1-butanol (C₆H₁₅NO), 2-(dimethylamino)ethanol (C₄H₁₁NO),2-(diethylamino)ethanol (C₆H₁₅NO), 2-di-n-butylaminoethanol (C₁₀H₂₃NO),N-ethyl-N-methylethanolamine (C₅H₁₃NO), 3-dimethylamino-1-propanol(C₅H₁₃NO), and 2-dimethylamino-2-methyl-1-propanol (C₆H₁₅NO).

In addition, the secondary cyclic diamine is a piperazine derivative.Examples of this piperazine derivative include compounds such aspiperazine (C₄H₁₀N₂), 2-methylpiperazine (C₅H₁₂N₂), and2,5-dimethylpiperazine (C₆H₁₄N₂), or a mixture thereof.

A description is provided for the absorbent liquid according to one ormore embodiments of the present invention. The absorbent liquid asdescribed herein is mainly different in that (b1) the tertiary linearmonoamine described earlier as a component is replaced by (b2) asterically hindered primary monoamine. Explanation of the sameconstituents as the aforementioned embodiments is omitted.

(b2) The sterically hindered primary monoamine is a compound representedby Formula (III) below. In Formula (III), R6 to R8 each represent ahydrogen group or a hydrocarbon group having 1 to 3 carbon atoms.

In particular, the primary monoamine is at least one selected from thegroup consisting of 2-amino-2-methyl-1-propanol (C₄H₁₁NO),2-amino-1-propanol (C₃H₉NO), 2-amino-1-butanol (C₄H₁₁NO),2-amino-3-methyl-1-butanol (C₅H₁₃NO), 1-amino-2-propanol (C₃H₉NO), and1-amino-2-butanol (C₄H₁₁NO).

The concentration of each of the components of the secondary linearmonoamine, the sterically hindered primary monoamine, and the secondarycyclic diamine is less than 30% by weight relative to the weight percentof the absorbent liquid. The total concentration of the secondary linearmonoamine, the sterically hindered primary monoamine, and the secondarycyclic diamine is preferably 70% by weight or less and more preferably10% by weight or more relative to the absorbent liquid. The upper limitof the concentration of the secondary linear monoamine (a) is less than30% by weight and is preferably 28% by weight or less. In addition, thelower limit of the concentration is more preferably 10% by weight ormore and is further preferably 15% by weight or more. The upper limit ofthe concentration of the sterically hindered primary monoamine (b2) isless than 30% by weight, preferably 28% by weight or less, and furtherpreferably 25% by weight or less. In addition, the lower limit of theconcentration is preferably 10% by weight or more and more preferably15% by weight or more. The upper limit of the concentration of thesecondary cyclic diamine (c) is less than 30% by weight, preferablywithin a range of 15% by weight or less, and more preferably 10% byweight or less. In addition, the lower limit of the concentration ispreferably 1% by weight or more and more preferably 5% by weight ormore.

The total concentration of the secondary linear monoamine and thesterically hindered primary monoamine relative to the absorbent liquidis 20 to 55% by weight and preferably 25 to 50% by weight. In addition,the concentration of the sterically hindered primary monoamine ispreferably the same as or less than the concentration of the secondarylinear monoamine relative to the weight percent of the absorbent liquid.In particular, the concentration of the sterically hindered primarymonoamine is preferably 0.6 to 1 in a weight ratio relative to theconcentration of the secondary linear monoamine. The concentration ofthe secondary cyclic diamine is preferably less than the concentrationsof the secondary linear monoamine and the primary monoamine relative tothe weight percent of the absorbent liquid.

As described above, if (a) the secondary linear monoamine, (b2) thesterically hindered primary monoamine, and (c) the secondary cyclicdiamine are at the above concentrations, an excellent CO₂ absorbingproperty of (a) the secondary linear monoamine and (c) the secondarycyclic diamine maintains the CO₂ absorbability of the absorbent liquid,and an excellent CO₂ stripping property of (a) the secondary linearmonoamine and (b2) the sterically hindered primary monoamine can improvethe CO₂ stripping performance of the absorbent liquid. Thus, it ispossible to reduce the amount of reboiler heat when regenerating theabsorbent liquid which has absorbed CO₂.

[CO₂ Removing Apparatus]

A description is provided for an embodiment of a CO₂ removing apparatusemploying the absorbent liquid having the above constitution withreference to FIG. 1. A CO₂ removing apparatus 1 illustrated in FIG. 1 atleast includes an absorption tower 10 and a regeneration tower 20. TheCO₂ removing apparatus in the figure is a diagram for its overalldescription, and its accompanying equipment is partially omitted.

The absorption tower 10 includes a lower packed section 11 a, an upperpacked section or a tray 11 b, and a liquid collector 11 c, contains anabsorbent liquid inside which absorbs CO₂ and/or H₂S in combustionexhaust gas, and includes coolers 12 and 13 outside. The regenerationtower 20 is provided downstream of the flow direction of the absorbentliquid which has absorbed CO₂ of the absorption tower 10, includes lowerpacked section 21 a and an upper packed section or a tray 21 b, andincludes a reboiler 22, a cooler 23, and a gas-liquid separator 24outside thereof. The regeneration tower 20 includes an absorbent liquidheat exchanger 25 between the absorption tower 10 and the regenerationtower 20. The regeneration tower 20 is configured to regenerate theabsorbent liquid which has absorbed CO₂ and/or H₂S by heat of thereboiler. In addition, the CO₂ removing apparatus 1 includes a coolingtower 30 upstream of the flow direction of combustion exhaust gas of theabsorption tower 10. The cooling tower 30 at least includes a packedsection 31 which causes cooling water and gas to come into contact witheach other, and includes a cooler 32 outside thereof which cools thecooling water.

[CO₂ Removing Method]

A description is provided for a CO₂ removing method according to one ormore embodiments of the present invention by explaining how the CO₂removing apparatus having the above configuration operates. The CO₂removing method according to the present embodiment includes a coolingstep, an absorption step, and a regeneration step.

At the cooling step, the combustion exhaust gas is supplied via a lineL₀ to the cooling tower 30 for cooling. In the cooling tower 30, thecombustion exhaust gas is brought into contact with the cooling waterfrom the cooler 32 in the packed section 31, cooled to a predeterminedtemperature, and introduced by a blower B₁ via a line L₁ to a lowerportion of the absorption tower 10. The temperature of the combustionexhaust gas to be cooled can be 30 to 40° C. as an example from theviewpoint of absorption performance. On the other hand, the coolingwater which has come into contact with the combustion exhaust gas isstored in the lower end portion of the cooling tower 30, and then iscirculated for use by a pump P₂ via a line L₂ and the cooler 32 to thecooling tower 30. At this step, if the amount of water in the combustionexhaust gas is small, since the cooling water is gradually reduced byhumidifying and cooling the combustion exhaust gas, cooling water issupplied via a not-illustrated water supply line provided in the lineL₂. If the amount of water in the combustion exhaust gas is large, sincethe amount of cooling water increases due to condensation of water inthe combustion exhaust gas by contact with the cooling water, the excessamount of water is discharged via a drain discharge line (not shown)provided in the line L₂.

Subsequently, at the absorption step, the combustion exhaust gasintroduced to the absorption tower 10 is brought into concurrentcontact, in the lower packed section 11 a, with the absorbent liquidintroduced via a line L₈ and having a predetermined concentration to bedescribed later. Thereby, the absorbent liquid absorbs CO₂ from thecombustion exhaust gas, removing CO₂ from the combustion exhaust gas. Inparticular, the lower packed section 11 a removes CO₂ by bringing theregenerated absorbent liquid (lean absorbent liquid) introduced via theline L₈ into contact with the combustion exhaust gas. This makes itpossible to remove 90% or more of the CO₂ in the combustion exhaust gas.The combustion exhaust gas from which CO₂ has been removed contains anamine absorbent and water vaporized under a high temperature due toexothermic reaction by CO₂ absorption. Thus, water and the amineabsorbent are condensed from the combustion exhaust gas into a liquid inthe cooler 13, and moreover brought into contact with a washing liquidof cooled condensation water in the upper packed section 11 b. Water andthe amine absorbent in the gas are condensed and recovered as a washingliquid. Thereby, water and the absorbent liquid are collected from thecombustion exhaust gas. The combustion exhaust gas from which CO₂ hasbeen removed is discharged from the tower top portion of the absorptiontower 10 to the line L₃. In addition, at this step, the washing liquidcontaining water and the absorbent liquid recovered from the combustionexhaust gas are collected in the liquid collector 11 c, the pump P₅circulates part of the washing liquid for use by cooling it with thecooler 13 on the line L₅, and an excess amount of washing liquid isadded to the lean absorbent liquid via the line L₆ in order to berecovered as the absorbent liquid. On the other hand, the absorbentliquid which has absorbed CO₂ (rich absorbent liquid) is collected inthe lower end portion located below the lower packed section 11 a. Therich absorbent liquid is sent by the pump P 4 via the line L 4 to theabsorbent liquid heat exchanger 25, followed by temperature rise byexchanging heat with the lean absorbent liquid to be described later,and then is sent to the regeneration tower 20.

The subsequent regeneration step is a step of regenerating the absorbentliquid by allowing the reboiler 22 to heat the CO₂-contain absorbentliquid to release CO₂ therefrom. To be more specific, the rich absorbentliquid supplied to the inside of the regeneration tower 20 flows downthe regeneration tower 20 and is stored in the lower portion liquidreservoir while emitting CO₂ due to endothermic reaction occurring inthe lower packed section 21 a, and the reboiler 22 raises thetemperature of the absorbent liquid having passed via the line L₇ byheat exchange with high-temperature saturated vapor supplied from theoutside. Thereby, the CO₂ in the absorbent liquid is released. Thesaturated steam introduced into the reboiler 22 condenses into saturatedwater by heat exchange with the absorbent liquid and is discharged fromthe reboiler 22. By stripping CO₂ from the absorbent liquid, the leanabsorbent liquid is obtained. As described above, a large amount ofthermal energy of steam is required in order to regenerate the absorbentliquid by stripping CO₂ in the regeneration tower 20, especially in thereboiler 22. In the reboiler 22, since the absorbent liquid according toone or more embodiments of the present invention has a high CO₂stripping performance as described above, it is possible to reduce theamount of saturated steam to be introduced into the reboiler 22 and thusto reduce the amount of reboiler heat duty by efficiently stripping CO₂.

The lean absorbent liquid from which CO₂ is stripped by heat supplied tothe reboiler 22 is introduced into the absorbent liquid heat exchanger25 via the line L₈. In the absorbent liquid heat exchanger 25, the leanabsorbent liquid is cooled by heat exchange with the rich absorbentliquid from the absorption tower 10 and is introduced into theabsorption tower 10. The temperature of the lean absorbent liquid to beintroduced into the absorption tower 10 can be adjusted with theabsorbent liquid heat exchanger 25 or a cooler 12 provided on the lineL₈ downstream thereof. On the other hand, CO₂ separated from theabsorbent liquid is subjected to the removal of accompanying absorbentliquid in the upper packed section 21 b located at the upper portion ofthe regeneration tower 20 by gas-liquid contact with reflux watersupplied via the line L₉. Then, the CO₂ is discharged to the line L₁₀from the upper portion of the regeneration tower 20. The cooler 23 coolsthe CO₂ gas including water vapor on the line L₁₀ to condense watervapor, followed by separation into CO₂ and reflux water in which watervapor is condensed in the gas-liquid separator 24. Then, the separatedCO₂ with high purity is discharged to the recovery line L₁₁ forrecovery, and the reflux water is refluxed by the pump P₉ to theregeneration tower 20 via the line L₉ for reuse.

The highly pure CO₂ recovered as described above can be injected in adepleted oil field for storage or can be preferably reused in order toenhance oil and natural gas recovery, for the purpose of synthesizingchemical products such as urea and methanol, and for general purposessuch as dry ice.

EXAMPLES

Hereinafter, one or more embodiments of the present invention isdescribed in detail using examples. The absorbent liquid for CO₂ and/orH₂S, and the apparatus and the method using the same are not limited bythe present examples.

1.1 Preparation According to Test Examples

An absorbent liquid of Test Example 1-1 was prepared by dissolving andmixing N-(butylamino)ethanol as the secondary linear monoamine,2-methylpiperazine as the secondary cyclic diamine, andN-methyldiethanolamine as the tertiary linear monoamine into water, eachless than 30% by weight. In addition, in the absorbent liquid, thesecondary linear monoamine and the tertiary linear monoamine werecontained at substantially the same concentrations in weight percent,and the secondary cyclic diamine was smallest in amount at aconcentration in weight percent. An absorbent liquid of Test Example 1-2was prepared in the same way as Test Example 1-1 except that thesecondary cyclic diamine was piperazine. An absorbent liquid of TestExample 1-3 was prepared in the same way as Test Example 1-1 except thatthe tertiary linear monoamine was N-butyldiethanolamine. An absorbentliquid of Test Example 1-4 was prepared in the same way as Test Example1-1 except that the secondary linear monoamine wasN-(ethylamino)ethanol, the secondary cyclic diamine was piperazine, andthe tertiary linear monoamine was N-ethyldiethanolamine. Test Example1-5 was prepared in the same way as Test Example 1-1 except that thesecondary linear monoamine was N-(ethylamino)ethanol, the secondarycyclic diamine was piperazine, and the tertiary linear monoamine was2-(diethylamino)ethanol. In addition, as a comparative example, anabsorbent liquid of Comparative Example 1-6 was prepared by dissolvingand mixing monoethanolamine (MEA) into water at 30% by weight. Table 1shows the component compositions of the test examples and thecomparative example.

TABLE 1 Table 1 Components of Absorbent Liquid (a) Secondary (b1)Tertiary (c) Secondary Linear Linear Cyclic Monoamine Monoamine DiamineTest N- N- 2-Methyl- Example (Butyl- Methyl- piperazine 1-1amino)ethanol diethanolamine Test N- N- Piperazine Example (Butyl-Methyl- 1-2 amino)ethanol diethanolamine Test N- N-Butyl- 2-Methyl-Example (Butyl- diethanolamine piperazine 1-3 amino)ethanol Test N-N-Ethyl- Piperazine Example (Ethyl- diethanolamine 1-4 amino)ethanolTest N- 2- Piperazine Example (Ethyl- (Diethylamino) 1-5 amino)ethanolethanol Primary Monoamine Com- Monoethanolamine parative Example 1-6

1.2 Evaluation of Reboiler Heat Duty Ratio

The reboiler heat duty was measured when the absorbent liquids of thetest examples and the comparative example were used, and the reboilerheat duty for the test examples was evaluated as a heat duty ratiocompared to the case of using the absorbent liquid of the comparativeexample. FIG. 2 illustrates the evaluation results.

As illustrated in FIG. 2, if the heat duty in the case of using theabsorbent liquid of Comparative Example 1-6 is set to 1, the reboilerheat duty ratios in the case of using the absorbent liquids of TestExamples 1-1 and 1-2 were less than 0.90, the reboiler heat duty ratiosin the case of using the absorbent liquids of Test Examples 1-3 and 1-4were about 0.90, and the reboiler heat duty ratio in the case of usingthe absorbent liquid of Test Example 1-5 was about 0.96. The aboveresults show that the absorbent liquids of Test Examples 1-1 to 1-5employing the secondary linear monoamine, the secondary cyclic diamine,and the tertiary linear monoamine can reduce the reboiler heat duty by10% or more compared to Comparative Example 1-6 employingmonoethanolamine. In particular, the absorbent liquids of Test Examples1-1 to 1-3 were shown to be able to reduce the reboiler heat duty by 10%or more.

1.3 Evaluation of Reboiler Heat Duty Ratio with Respect to ComponentConcentration

Subsequently, an absorbent liquid was prepared by dissolving and mixingN-(butylamino)ethanol as the secondary linear monoamine andN-butyldiethanolamine as the tertiary linear monoamine into water withthe total concentration of the two components at 40% by weight. Then,investigation was carried out for the reboiler heat duty ratio in thecase of varying the concentrations of the components while keeping thistotal concentration constant at 40% by weight. The reboiler heat dutyratio was calculated such that the result of using the absorbent liquidof Test Example 1-6 is 1. FIG. 3 shows the evaluation results.

As illustrated in FIG. 3, if the component concentration of thesecondary linear monoamine was 10% by weight, the reboiler heat dutyratio was about 0.88, and the reboiler heat duty ratio decreased whilethe component concentration increased toward about 20% by weight. Thereboiler heat duty ratio in the case of the component concentration ofabout 20% by weight was about 0.87, the reboiler heat duty ratio in thecase of about 25% by weight was about 0.88, and the reboiler heat amountratio increased toward about 0.90 while the component concentrationincreased toward 30% by weight. In addition, if the componentconcentration of the tertiary linear monoamine was 10% by weight, thereboiler heat duty ratio was about 0.90, the reboiler heat duty ratio inthe case of about 15% by weight was about 0.88, and the reboiler heatduty ratio decreased while the component concentration increased toward20% by weight. The reboiler heat duty ratio in the case of the componentconcentration of about 20% by weight was about 0.87, and the reboilerheat duty ratio increased toward about 0.88 while the componentconcentration increased toward 30% by weight.

2.1 Preparation of Test Examples

An absorbent liquid of Test Example 2-1 was prepared by dissolving andmixing N-(butylamino)ethanol as the secondary linear monoamine,2-methylpiperazine as the secondary cyclic diamine, and2-amino-2-methyl-1-propanol as the sterically hindered primary monoamineinto water, each less than 30% by weight. In addition, in the absorbentliquid, the secondary linear monoamine and the primary monoamine werecontained at substantially the same concentrations in weight percent,and the secondary cyclic diamine was smallest in amount at aconcentration in weight percent, as in the case of the test examplesdescribed earlier. An absorbent liquid of Test Example 2-2 was preparedin the same way as Test Example 2-1 except that the secondary cyclicdiamine was piperazine. An absorbent liquid of Test Example 2-3 wasprepared in the same way as Test Example 2-1 except that the secondarylinear monoamine was N-(ethylamino)ethanol. An absorbent liquid of TestExample 2-4 was prepared in the same way as Test Example 2-1 except thatthe secondary cyclic diamine was piperazine and the secondary linearmonoamine was N-(ethylamino)ethanol. In addition, as a comparativeexample, an absorbent liquid of Comparative Example 2-5 was prepared bydissolving and mixing monoethanolamine (MEA) into water at 30% byweight. Table 2 shows the component compositions of the test examplesand the comparative example.

TABLE 2 Table 2 Components of Absorbent Liquid (a) Secondary (b2) (c)Secondary Linear Primary Cyclic Monoamine Monoamine Diamine Test N-2-Amino- 2-Methyl- Example (Butyl- 2-methyl-1- piperazine 2-1amino)ethanol propanol Test N- 2-Amino-2- Piperazine Example (Butyl-methyl- 2-2 amino)ethanol 1-propanol Test N- 2-Amino-2- 2-Methyl-Example (Ethyl- methyl- piperazine 2-3 amino)ethanol 1-propanol Test N-2-Amino-2- Piperazine Example (Ethyl- methyl- 2-4 amino)ethanol1-propanol primary monoamine Com- monoethanolamine parative Example 2-5

2.2 Evaluation of Reboiler Heat Duty Ratio

The reboiler heat duty was measured when the absorbent liquids of TestExamples 2-1 to 2-4 and Comparative Example 2-5 were used, and thereboiler heat duty for the test examples was evaluated as a heat dutyratio compared to the case of using the absorbent liquid of thecomparative example in the same way as the evaluation of heat duty ratiodescribed earlier. FIG. 4 illustrates the evaluation results.

As illustrated in FIG. 4, in comparison with the absorbent liquid ofComparative Example 2-5, the reboiler heat duty ratio in the case ofusing the absorbent liquid of Test Example 2-1 was about 0.90, thereboiler heat duty ratios in the case of using the absorbent liquids ofTest Examples 2-2 and 2-4 were less than 0.93, and the reboiler heatduty ratio in the case of using the absorbent liquid of Test Example 2-3was about 0.98.

The above results show that the absorbent liquids of Test Examples 2-1to 2-4 employing the secondary linear monoamine, the secondary cyclicdiamine, and the sterically hindered primary monoamine can reduce theamount of reboiler heat by about 10% compared to the absorbent liquid ofComparative Example 2-5 employing monoethanolamine. In particular, theabsorbent liquid of Test Example 2-1, 2-2, or 2-4 was shown to be ableto reduce the amount of reboiler heat by 7 to 10%.

2.3 Evaluation of Reboiler Heat Duty Ratio with Respect to ComponentConcentration

Subsequently, an absorbent liquid was prepared by mixingN-(butylamino)ethanol as the secondary linear monoamine and2-amino-2-methyl-1-propanol as the sterically hindered primary monoamineinto water with the total concentration of the two components at 48% byweight. Then, investigation was carried out for the reboiler heat amountratio in the case of varying the concentrations of the components whilekeeping this total concentration constant at 48% by weight in a range of2-amino-2-methyl-1-propanol 30% by weight or less. The reboiler heatamount ratio was calculated such that the result of using the absorbentliquid of Test Example 2-5 is 1. FIG. 5 shows the evaluation results.

As illustrated in FIG. 5, if the component concentration of thesecondary linear monoamine was about 10% by weight, the reboiler heatduty ratio was about 0.92, the reboiler heat duty ratio decreased toabout 0.90 while the component concentration increased toward about 20%by weight, and the reboiler heat duty ratio increased to about 0.93while the component concentration increased toward 30% by weight. Inaddition, if the component concentration of the primary monoamine was10% by weight, the reboiler heat amount ratio was about 0.94, thereboiler heat amount ratio decreased to about 0.92 while the componentconcentration increased toward about 20% by weight, the reboiler heatduty ratio decreased to about 0.90 while the component concentrationincreased toward about 28% by weight, and the reboiler heat duty ratiorapidly increased to about 0.92 while the component concentrationincreased toward 30% by weight.

REFERENCE SYMBOL LIST

-   1 CO₂ removing apparatus-   10 collection tower-   11 a lower packed section-   11 b upper packed section-   11 c liquid collector-   12, 13, 23, 32 cooler-   20 regeneration tower-   22 reboiler-   24 gas-liquid separator-   25 absorbent liquid heat exchanger-   30 cooling tower-   31 packed section

Although the disclosure has been described with respect to only alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that various other embodiments maybe devised without departing from the scope of the present invention.Accordingly, the scope of the invention should be limited only by theattached claims.

The invention claimed is:
 1. An absorbent liquid which absorbs at leastone of CO₂ and H₂S from a gas, consisting of: water; a secondary linearmonoamine; a sterically hindered primary monoamine; and a secondarycyclic diamine, wherein a concentration of each of the secondary linearmonoamine, the tertiary linear monoamine or the sterically hinderedprimary monoamine and the secondary cyclic diamine is less than 30% byweight, and wherein the secondary linear monoamine is at least oneselected from the group consisting of: N-(methylamino)ethanol,N-(ethylamino)ethanol, N-(propylamino)ethanol, andN-(butylamino)ethanol, and wherein the sterically hindered primarymonoamine is at least one selected from the group consisting of:2-amino-1-propanol, 2-amino-1-butanol, 2-amino-3-methyl-1-butanol,1-amino-2-propanol, and 1-amino-2-butanol.
 2. The absorbent liquidaccording to claim 1, wherein the concentration of the secondary cyclicdiamine is less than the concentration of the secondary linear monoamineand is less than the concentration of the sterically hindered primarymonoamine in weight percent relative to the absorbent liquid.
 3. Theabsorbent liquid according to claim 2, wherein the concentration of thesecondary linear monoamine is from 10% by weight to 28% by weight. 4.The absorbent liquid according to claim 3, wherein the absorbent liquidconsists of: the water; the secondary linear monoamine; the steric allyhindered primary monoamine; and the secondary cyclic diamine, andwherein the concentration of the sterically hindered primary monoamineis 20% by weight or more.
 5. The absorbent liquid according to claim 1,wherein the secondary cyclic diamine is a piperazine derivative.
 6. Theabsorbent liquid according to claim 5, wherein the secondary cyclicdiamine is piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, or amixture thereof.
 7. The absorbent liquid according to claim 1, whereinthe concentration of the sterically hindered primary monoamine is thesame as or less than the concentration of the secondary linear monoaminein weight percent relative to the absorbent liquid.
 8. The absorbentliquid according to claim 1, wherein a total concentration of thesecondary linear monoamine and sterically hindered primary monoamine is20 to 55% by weight.
 9. The absorbent liquid according to claim 1,wherein a total concentration of the secondary linear monoamine, thesterically hindered primary monoamine, and the secondary cyclic diamineis 70% by weight or less.
 10. The absorbent liquid according to claim 1,wherein the concentration of each of the secondary linear monoamine, andthe sterically hindered primary monoamine, is 10% by weight or more. 11.An apparatus for removing at least one of CO₂ and H₂S from a gas,comprising: an absorption tower which includes the absorbent liquidaccording to claim 1; and a regeneration tower which regenerates theabsorbent liquid containing the at least one CO₂ and H₂S absorbedtherein by heat of a reboiler.
 12. A method for removing at least one ofCO₂ and H₂S from a gas, the method comprising: absorbing at least one ofCO₂ and H₂S by bringing the absorbent liquid according to claim 1 intocontact with the gas; and regenerating the absorbent liquid containingthe at least one of CO₂ and H₂S absorbed therein by heat of a reboiler.